1. Field of the Invention
The present invention relates to a field/frame conversion circuit for use in a magnetic recording/reproducing apparatus adapted to record and reproduce still images or the like and, in particular, to a field/frame conversion circuit in which, when a field signal reproduced from a magnetic recording medium is delayed one-half of a horizontal scan period in order to provide a frame signal, the field signal is delayed in a high frequency bandwidth.
2. Description of the Prior Art
In forming a television picture image, it is well known that an interlaced scanning is employed in a horizontal scanning in order to reduce flicker to the eyes. In the NTSC system, this interlaced scanning is a [2 to 1] interlaced scanning method. In the [2 to 1] interlaced scanning method, a picture (that is, a frame) is structured by superposing two rough pictures (that is, two fields) on each other, each of the fields being formed by means of one vertical scanning. Accordingly, the number of repetitions of frames is 30 times per second and the number of the field repetitions is 60 times per second. Also, odd fields and even fields are delayed from each other by 0.5 H where 1 H represents a horizontal scanning period.
By the way, when recording video signals in a recording medium such as a magnetic tape, a magnetic disc or the like, various kinds of recording methods are conventionally employed. Among them, for example, there is a method in which a video signal is divided into two signal components, that is, a brightness signal and a chroma signal, and the two divided signals are processed in a given manner and are then frequency modulated, before they are recorded into the magnetic recording medium.
For reproduction in such recording method, there is employed a so-called field/frame conversion technique in which the strong vertical correlation of the video signal is used to scan the same recorded portion twice so that a frame signal can be created from one kind of field signal. In the field/frame conversion technique, since the field can not be divided into odd fields and even fields simply by repeating the same field signal there are formed two kinds of signals, that is, a signal that is delayed 0.5 H from the field signal (0.5 H delay signal) and a signal that is not delayed (non-delay signal), and these two kinds of signals are switched so as to provide the odd and even fields.
In a field/frame conversion circuit to realize the above-mentioned field/frame conversion system, a 0.5 H delay line is used to to provide the signal that is delayed 0.5 H from the field signal and a charge coupled device (CCD) is employed in the 0.5 H delay line. The delay line using the CCD is not so good in S/N (signal-to-noise ratio) the frequency characteristic thereof does not exhibit a wide bandwidth and, therefore, the brightness signal and the chroma signal must be converted separately into the frame signal. This requires two kinds of different delay lines (that is, glass delay line and CCD delay line) which are different from each other in characteristic. The difference in characteristic between the glass and CCD delay lines results in a great difference of delay time between the brightness and chroma signals which can produce color discrepancies.
Also, in the above-mentioned field/frame conversion circuit, when the chroma signal is demodulated, there is employed a bandwith which is not suitable to demodulate the chroma signal, resulting in a distorted chroma signal.
It may be considered that, instead of the above-mentioned two different kinds of delay lines, the one CCD delay line is used as the delay line for the brightness and chroma signals and at least the brightness signal is delayed 0.5 H after it is demodulated.
However, in this case, there may be produced a slight level difference between the non-delay brightness signal and the 0.5 H-delayed brightness signal, which results in a flicker of 30 Hz. In order to prevent the production of such flicker, a complicated circuit configuration must be used.